The present invention relates to an inverter apparatus.
As a control apparatus for an AC motor such as an induction motor subjected to variable-speed control, a voltage-fed multiple inverter apparatus of a PWM control scheme in FIG. 1 is known.
The apparatus shown in FIG. 1 is an inverter in which two single-phase inverters 12 are connected in series to form one phase, and three sets of such single-phase inverters are star-connected. A motor 13 is driven by using output terminals U, V, and W. In this case, a transformer 11 supplies insulated three-phase power to the AC input terminals of the respective single-phase inverters. Each single-phase inverter 12 will be described in detail with reference to FIG. 2. Referring to FIG. 2, three-phase AC power is converted into DC power by a three-phase bridge rectifier 14, and the current is smoothed by a smoothing capacitor 15. This DC power is converted into an AC power by a single-phase bridge inverter 16. In this conversion, the DC power is converted into an AC power of a desired voltage and frequency by so-called PWM control.
According to the arrangement shown in FIG. 1, an output three times higher than the voltage output from each single-phase inverter can be obtained, and hence a large-capacity inverter can be formed. By shifting the PWM control timings of two single-phase inverters constituting one phase from each other, the PWM frequency appearing at the output terminal is doubled. As a consequence, each voltage step is reduced to 1/2 from the viewpoint of the overall phase, and an output similar to a sine wave can be obtained.
In the above multiple inverter, a PWM signal can be obtained by shifting the phases of carrier signals.
FIG. 3 shows examples of output voltage waveforms. By alternately switching output voltages U1 and U2 from two single-phase inverters, a waveform more similar to a sine wave can be obtained as a whole. In addition, by shifting the phase of the secondary winding of the transformer 11, input harmonics can be reduced. Consider two sets of single-phase inverters on the upper and lower stages. Since the single-phase inverters on each stage output three-phase power, the power is smoothed from the viewpoint of each stage, and a so-called "12-pulse arrangement" with the phases of the windings of the transformer being shifted by 30.degree. is formed. This arrangement can reduce input harmonics.
FIG. 1 shows an example of two stages. Obviously, however, an improved result can be obtained with three or more stages.
However, single-phase inverters require a larger number of parts than a three-phase inverter, and have power pulsations having a frequency twice the output frequency at the DC portion. Since the peaks of power pulsations must be considered, the use efficiency of the rectifier of each single-phase inverter is lower than that of the rectifier of the three-phase inverter. In addition, large current ripples are produced in the smoothing capacitor of each single-phase inverter. Furthermore, since the current ripples in the smoothing capacitor greatly affect the service life of the capacitor, a sufficient design margin must be ensured. This greatly increases the cost of the inverter apparatus.
In order to process regeneration power from a load, DC power in all the single-phase inverters must be consumed by using resistors, or a special power converting circuit is required, resulting in problems in terms of both cost and size. Furthermore, as the number of stages of single-phase inverters decreases, the input harmonics of currents increase.
The present invention is therefore made in consideration of the above problems, and has as its object to provide an inverter apparatus which can attain increases in the capacity and voltage of the inverter by combining a three-phase inverter and single-phase inverters, improve reliability and cost performance by reducing the number of parts, and realizing a reduction in input harmonics and a regeneration function at a low cost.